This invention relates to a class of austenitic stainless steels provided with high resistance against oxidation at high temperatures especially when they undergo cyclic heating to high temperatures in oxidative atomosphere.
Recently in every country, as one of the measures for prevention of atmospheric pollution, regulations on control of automobile exhaust gases have become more and more severe. In order to comply with such regulations, various methods and means are being searched for. The immediate necessity is development of a complete system for purifying auto exhaust gases of the conventional internal combustion engines. As the means therefor, thermal reactor, after-burner, catalytic converter, etc. have been developed and they are used singly or in combination. These apparatuses are exposed to hot burning gases and are cyclically heated to high temperatures over 1100.degree. C, often 1200.degree. C or higher. So, the materials for these apparatuses must have high resistance to oxidation and corrosion at high temperatures and resistance to scaling as well as high temperature strength.
Typical materials that have been considered to be suitable for those apparatuses are ferritic Fe-Cr-Al alloys such as JIS (Japanese Industrial Standard) FCH-1 (25Cr-5Al), JIS FCH-2 (19Cr-3Al), etc. and austenitic stainless steels such as Type 310 steels, etc. These materials are appreciated because of their high temperature characteristics, workability and low cost. However, the ferritic alloys are remarkably inferior to the austenitic stainless steels in high temperature strength and are easily deformed when subjected to cyclic heating, and are inferior in weldability and workability, too, although they have good resistance to oxidation scaling and gas corrosion because of formation of protective film of alumina. The austenitic stainless steels have good high temperature strength, but they are inferior to the ferritic alloys in high temperature oxidation resistance and scaling resistance. Especially Type 310 steels retain austenitic structure even in the welded state, and therefore easily suffer hot cracking in welding. Further they are considerably expensive and cannot be used freely.
As the stainless steels with good high temperature oxidation resistance, high silicon heat-resisting austenitic stainless steels are known as AISI 302B (18Cr-9Ni-2.5Si), AISI 314 (25Cr-20Ni-2Si), DIN 4828 (20Cr-12Ni-2Si), etc. These steels have excellent oxidation resistance at temperatures of 1000.degree. to 1100.degree. C, but are easily oxidized at temperatures over 1100.degree. C and oxide scales easily spall and peel off. In order to improve properties of these steels, addition of rare earth metals such as Y, Ce, La etc. has been tried, but has not brought about satisfactory results.
It was known that combined addition of Si and Al or Si, Al and Ca gives good oxidation resistance to steels of these types. For instance, in U.S. Pat. No. 3,729,308, a class of stainless steels containing up to about 0.25% C, 0.3% - 1.5% Si, up to 2% Mn, 15 - 23% Ni, 17 - 23% Cr, up to 0.5% Al, up to 0.05 Ce and up to 0.5% Ti is disclosed. (Percentages are all by weight.) In U.S. Pat. No. 3,837,846, a class of steels or rather super alloys containing 0.01 - 0.10% C, about 0.5% Si, an effective amount of Mn, 15 - 45% Ni, 16 - 35% Cr, 0.001 - 0.008% Ca, 0.1 - 1.5% Al, and others is disclosed. In Japanese Patent Application Publication No. 32328/71, a class of steel containing 0.05 - 0.4% C, 0.2 - 2% Si, 0.5 - 5% Mn, 8 - 25% Ni, 14 - 30% Cr, 0.003 - 0.5% Ca is disclosed. In Japanese Patent Application Publication No. 32330/71, a class of steels containing 0.05 - 0.4% C, 0.2 - 2% Si, 0.5 - 5% Mn, 8 - 25% Ni, 14 - 30% Cr, 0.003 - 0.5% Ca, and 0.003 - 0.5% Mg is disclosed. All these steels contain Si in an amount not exceeding 2%, and all of them are still inferior in scaling resistance at temperatures over 1100.degree. C.
Similar steels are disclosed in U.S. Pat. No. 2,553,330, which does not teach anything about high temperature scaling resistance, especially scaling resistance when the steel is cyclicly heated to temperatures over 1100.degree. C.
U.S. Pat. No. 2,687,954 discloses Incolloy 800 type alloys containing 0.01 - 1.0% Al, 0.001 - 0.20% Ca, up to 0.50% rare earth metal, which may further contain up to 0.25% C, 0.20 - 3.0% Si and 0.02 - 4.0% Mn. It is well known that alloys of this type is very susceptible to high temperature cracking, especially when their Si content is high. So the alloys of this type is neither workable nor weldable. And they are very expensive materials. We ourselves found that combined addition of Si and Al or Si, Al and rare earth metals gives rather inexpensive stainless steel materials which are provided with good high temperature strength comparable with that of Type 310 steels and that are superior thereto in oxidation resistance and scaling resistance, too. (Japanese Patent Application No. 93354/73 (Laying-Open Publication No. 46509/75) and Japanese Patent Application No. 106948/73 (Laying-Open Publication No. 57913/75))
In the course of the study, we now have found that combined incorporation of 2.56 - 4.0% of Si and a small amount of an alkali earth metal such as Ca in the austenitic stainless steel greatly improves oxidation and scaling resistance retaining high temperature strength and workaability of said steel. Also, it has been learned that single or combined addition of Al, at least one rare earth metal such as Y, La, Ce, etc. and at least one element selected from the class consisting of Nb, Ta, Ti, Zr, and Hf in addition to an alkaline earth metal further improves the high temperature properties of said steel and we have created this invention.
Prior to our finding, austenitic stainless steels containing more than 2.5% Si and Ca was not known. Because it was well known that austenitic steels containing such a high level of Si were highly susceptible to high temperature cracking and could not be welded.